Document that osspd requirement for using OSS under Linux

Without SDL, wxSound doesn't work out of the box on most (all?)
contemporary Linux systems, so at least document this.

See #14899.

interface/wx/sound.h

@@ -17,8 +17,11 @@
     This class represents a short sound (loaded from Windows WAV file), that
     can be stored in memory and played.
 
-    Currently this class is implemented on Windows and Unix (and uses either
-    Open Sound System or Simple DirectMedia Layer).
+    Currently this class is implemented on Windows and Unix and can use either
+    Open Sound System (OSS) or Simple DirectMedia Layer (SDL) under the latter.
+    Notice that OSS is not provided any more by most, and maybe even all,
+    Linux systems in 2017 and osspd (OSS Proxy Daemon) package typically needs
+    to be installed to make it work.
 
     @library{wxadv}
     @category{media}